Natural gas has become a major fuel source in the world economy. For fuel deficient regions, the drawback of natural gas as a fuel is the problem in transporting the gas economically from the production site of the gas, usually in remote regions of the world, to the utilization sites, usually the highly industrialized or populated areas of the world. In order to make natural gas a more viable fuel, producers of the gas have utilized large liquefaction plants to cool and condense the produced natural gas for more viable long distance shipment to the end user. Liquefaction requires enormous energy in order to reduce the temperature of the natural gas under cryogenic conditions generally to a temperature of approximately -259.degree. F. In order to make a liquefaction scheme economical, it is necessary to process huge volumes of natural gas under the most efficient conditions possible. The efficiency of a liquefaction process is dependent upon various factors, several of which are the selection of cryogenic machinery available as stock items for such a facility and ambient conditions which exist at the site of the base load liquefaction plant.
Various schemes have been set forth in the prior art for achieving the cold temperatures necessary for natural gas liquefaction. In U.S. Pat. No. 4,225,329 a process is set forth wherein the feed natural gas is initially cooled in one refrigeration system and is subsequently cooled in a cascade refrigeration system whereby the natural gas cools itself by a series of flash stages wherein the rapid reduction in pressure of the natural gas provides cooling with the separation of a liquid phase from a gaseous phase. The gaseous phase is recycled for recompression and introduction into the feed gas stream. A portion of the flashed gas is rewarmed for use as plant fuel. The refrigeration system of this process achieves a partial liquefaction temperature of the natural gas of -141.degree. F. It requires a series of flash stages wherein the natural gas itself provides its own refrigeration in order to cool the liquefied natural gas to the typical storage temperature of -259.degree. F.
The prior art has also sought methods for shifting compression load between dual closed refrigeration cycles in a liquefaction plant. In U.S. Pat. No. 4,404,008 interstage cooling with a propane precool refrigeration cycle of a mixed component subcool refrigeration cycle is performed in order to balance the compressor driver requirements of both the precool and the subcool cycles. This allows the driver motors of a given liquefaction plant to be of the same size and configuration as desired by most plant owners and operators.
A two, closed refrigeration cycle LNG plant is set forth in U.S. Pat. No. 3,763,658 wherein cooling load is exchanged between a propane precool cycle and a mix component subcool cycle.
A typical commercial installation for an LNG plant using only a single, mix component refrigeration cycle is exemplified by the N.E.E.S. installation near Boston, Mass. which went on line in the 1970s.
The present invention overcomes the problem of mismatched compressor drivers, inefficient liquefaction operation and high equipment capital costs by a unique process flowscheme as set forth below.